While treating hard water, a bed of ion exchange resin or other material in a water conditioner removes calcium and magnesium ions from the water and replaces them with sodium ions. As the hard water passes through the bed, it exchanges these hard water ions with sodium in the first soft resin it meets, creating a front or wave of ion-exchange activity called the reaction zone. The bed becomes ineffective for softening and must be periodically regenerated when the amount of available sodium is depleted and the ion-exchange material is saturated with calcium and magnesium. Water treatment is then suspended while the ion-exchange material is regenerated in a multi-step process to flush the calcium and magnesium ions from the resin and restore the sodium level.
A series of steps is used to replace the hard water ions with sodium ions, making the ion-exchange material active again for water conditioning. Typically, the bed is first backwashed, by reversing the flow of the incoming water, to remove sediment and loosen the bed. Next, the bed is contacted with a downflowing brine solution, whereby the ion-exchange material takes sodium ions from the high concentration brine solution and displaces the calcium and magnesium ions into the brine and out to waste. When an optimum amount of brine solution has been delivered, rinsing continues until virtually all traces of the brine solution and the unwanted hard water ions in it are discharged from the bed. After being rinsed to remove residual brine, the bed has been restored to the sodium state, known as soft resin, and is then returned to service treating hard water.
Preparation of the brine solution typically takes place in a brine tank that is kept separate from the resin tank. The brine tank, which contains a supply of salt, is filled with a measured amount of water to form a saturated salt solution. The salt supply must be replaced periodically due to depletion after repeated regenerations. If the salt level is too low to make a brine solution of a given strength, there will be an insufficient sodium level to drive the exchange of calcium and magnesium ions and the resin will not effectively treat the hard water when it is placed back in service.
Most modern water conditioners such as water softeners and the like use electronic controllers to perform calculations, monitor sensors, direct timing and control valves during the various process steps. Some newer, more sophisticated water conditioners use electronics to schedule the next regeneration cycle based on one or more inputs. The input data includes, for example, information from timers, flow meters, stored historical data on water usage and the like. Many control sequences have been devised to determine the sequence and duration of the various steps required during regeneration of a water conditioner. In a simple regeneration sequence, each step is a fixed length of time, regardless of the degree of calcium and magnesium saturation of the resin. To ensure that the bed was fully regenerated, the duration of each step would have to be at least the time necessary for that process step, assuming that the resin was completely saturated with hard water ions at the start of the regeneration. Using this technique, the same amount of time and brine are used regardless of whether the resin is 10% saturated, 40% saturated or 90% saturated, resulting in a waste of time and salt when the resin is less than saturated with hard water ions.
When designing a regeneration control sequence, it is preferable to minimize the duration of the regeneration process for a number of reasons. While the unit is being regenerated, it is out of service for softening water. Most consumers want their water conditioner to provide soft water on demand, even very late at night or very early in the morning. Reducing the amount of time the unit is out of service decreases the probability that soft water will be unavailable when needed. Using less salt and water for regeneration reduces the cost of operation. There is also a need to minimize the amount of brine discharged from the water conditioner to the environment. Reducing the duration of the brine cycle helps to minimize the use of brine, thereby reducing the impact on the environment.
In U.S. Pat. No. 5,699,272, the duration of a rinse cycle is determined using the difference in voltage between a sensor probe and a reference probe by looking for three distinct states. The first state occurs when the bed is totally surrounded by sodium ions at the beginning of the brine/slow rinse cycle or step, indicating that the brine has filled the bed. As the delivery of brine stops and the rinse water washes the sodium away, a front moves through the bed with a high sodium concentration ahead of it and a low sodium concentration behind it. The second stage occurs when the front is between the sensor probe and the reference probe, indicating that the brine solution is being rinsed from the bed. The third stage occurs when the front has passed the reference probe, both sensors will be in the low sodium solution, signaling that the rinse can be discontinued.
None of the know prior art regeneration schemes consider the effects of manufacturing variations or fouling of the probes or sensors over time. When differences between two probes or between a probe and a reference value are used to determine the end of the cycle, changes can produce the same difference in values as the passing of a front. Further, sensors can become covered with sediment, scale, rust deposits or otherwise fouled, making the sensor less sensitive over time to the changes that surround it. As sensor sensitivity drops, the differences in readings become less distinct and impact the ability to correctly detect the beginning or end of a process step. As a result, the unit can fail to recognize the need to regenerate or it regenerates more frequently than is necessary.
Additionally, plating of the sensors causes the comparator to signal for premature regeneration because the impedance steadily increases. As a result, reserve capacities are increased and softener efficiency is decreased, leading to a waste of water and salt.
Further, the prior art use sensor readings in fixed comparisons or compare them to predetermined values. It is difficult to compensate for a replacement sensor that gives slightly different impedance readings due to manufacturing differences. The available software cannot account for sensors that have become plated from years of exposure to minerals in a flowing water environment. The fixed or predetermined values may take into account initial states of some of these variables, but cannot compensate for changes over time.
Thus, there is a need for a method for determining the duration of the steps in the process cycle of a water conditioner that maintains accuracy over long periods of time. The method should accurately determine the termination of the service step or a brine/slow rinse step in spite of fouling or replacement of one or more of the sensor probes.